I. Field of the Invention
This invention relates generally to the design of central and peripheral sensors used in conjunction with cardiac pacemakers, monitors, drug delivery systems and other cardiac measuring devices and more particularly to a sensor for the determination of cardiac or thoracic volumes which is not dependent upon the presence of a carrier signal. It accomplishes this by determining volume as a function of the slope of capacitor discharge by measuring the trailing edge voltage (TEV) or current of the generator pulse as an indicator of load.
II. Discussion of the Prior Art
Intra-cardiac and thoracic volume measurements provide useful data for diagnosis and adaptive rate pacing. Those systems which measure intra-cardiac or intra-thoracic volume changes using impedance plesmography require the delivery of an AC or pulsed constant current carrier signal to obtain such an impedance measurement. This method has several disadvantages, including high battery drain, and somewhat complex detection algorithms.
An example of an impedance-based system is that set out in U.S. Pat. No. 4,868,987 to Salo. It uses a relatively high frequency AC carrier signal applied between spaced electrodes disposed in a beating heart. The inflow and outflow of blood in the cardiac chamber results in changes in impedance that effectively modulates the AC carrier and this modulated signal is sensed between the electrodes. This modulated carrier signal is processed to remove electrical artifacts and is then demodulated to remove the carrier frequency component. This yields an envelope which is proportional to instantaneous ventricular volume, from which a current may be derived and injected into the timing circuit of a demand-type pacemaker for changing its escape interval.
A carrier signal dependent device such as the Salo method and apparatus requires a battery to drive an oscillator which provides the necessary alternating current carrier signal to the drive electrodes. This consumes battery power, thus requiring more frequent periodic replacement or recharging.